Potent V2 vasopressin antagonists with structural changes at their C-terminals

Oxytocin and vasopressin agonists and antagonists as research tools and potential therapeutics

We recently reviewed the status of peptide and nonpeptide agonists and antagonists for the V(1a), V(1b) and V(2) receptors for arginine vasopressin (AVP) and the oxytocin receptor for oxytocin (OT). In the present review, we update the status of peptides and nonpeptides as: (i) research tools and (ii) therapeutic agents. We also present our recent findings on the design of fluorescent ligands for V(1b) receptor localisation and for OT receptor dimerisation. We note the exciting discoveries regarding two novel naturally occurring analogues of OT. Recent reports of a selective VP V(1a) agonist and a selective OT agonist point to the continued therapeutic potential of peptides in this field. To date, only two nonpeptides, the V(2) /V(1a) antagonist, conivaptan and the V(2) antagonist tolvaptan have received Food and Drug Administration approval for clinical use. The development of nonpeptide AVP V(1a), V(1b) and V(2) antagonists and OT agonists and antagonists has recently been abandoned by Merck, Sanofi and Pfizer. A promising OT antagonist, Retosiban, developed at Glaxo SmithKline is currently in a Phase II clinical trial for the prevention of premature labour. A number of the nonpeptide ligands that were not successful in clinical trials are proving to be valuable as research tools. Peptide agonists and antagonists continue to be very widely used as research tools in this field. In this regard, we present receptor data on some of the most widely used peptide and nonpeptide ligands, as a guide for their use, especially with regard to receptor selectivity and species differences.

Impact of the Merrifield solid phase method on the design and synthesis of selective agonists and antagonists of oxytocin and vasopressin: a historical perspective

This tribute to Bruce Merrifield traces the author's fortuitous path in 1964 from Vincent du Vigneaud's laboratory to the laboratory of D. W. Woolley to learn the solid phase method and then to his first faculty position in the Department of Biochemistry, McGill University, Montreal in 1965. It recalls the key roles played from early 1966 to July 1967 by Bruce Merrifield, John Stewart, Arnold Marglin, Herb Takashima, and Vincent du Vigneaud in providing key advice to the author's efforts to use the solid phase method to synthesize oxytocin; while simultaneously the du Vigneaud and Merrifield laboratories were collaborating on the solid phase synthesis of deamino-oxytocin. Both syntheses were published in the same issue of the Journal of American Chemical Society in 1968. Also described is how this breakthrough impacted the author's scientific career: by leading to highly productive collaborative studies, initially with Wilbur H. Sawyer and subsequently with others, on the design and synthesis of selective agonists, antagonists, and radioiodinated ligands for oxytocin and vasopressin receptors. These syntheses were greatly facilitated by the contributions of highly talented graduate students, research technicians, and visiting peptide chemists from Hungary, England, Poland, Bulgaria, and China. Many of these peptides have become very valuable pharmacological tools in studies on the peripheral and central effects of oxytocin and vasopressin: further attesting to the profound impact of the solid phase method as the cornerstone for all the discoveries, which he and his collaborators and coworkers have made over the past 40 years.

Targeted tyrosine iodination in a multi-tyrosine vasopressin analog

Iodination of the conserved 2-tyrosine (Tyr(2)) residue in the pressin and tocin rings of arginine- or lysine-vasopressin (AVP or LVP), and oxytocin, respectively, impairs binding to their respective receptors. Synthetic antagonists that have their Tyr(2) either replaced by another amino acid or irreversibly blocked by an O-methyl or O-ethyl ether, but have, instead, an iodinatable phenol moiety outside the pressin/tocin ring, are used for radiolabeling. We explored another approach to avoid iodinating Tyr(2) by capping this residue with a reversible O-acetyl group, incorporated during peptide synthesis. The O-acetyl-Tyr(2) LVP peptide, with a free iodinatable tyrosine attached to the epsilon-amine of 8-lysine, is iodinated at a neutral pH and purified by reverse-phase high-pressure liquid chromatography (HPLC) at an acidic pH, conditions under which the O-acetyl groups are stable. Deacetylation with hydroxylamine is selective, and leaves intact the disulfide bridge. The marked shortening of the HPLC retention time after deblocking produces a chemically homogeneous label, iodinated exclusively on the free tyrosine residue attached to the epsilon-amine of LVP. Hitherto, this (125)I labeled vasopressin agonist could be obtained only in low yield, via conjugation labeling with iodinated N-t-Boc-tyrosine succinimidyl ester. This fully reversible tyrosine protection strategy does not require special equipment, and retains the conserved Tyr(2), typical of vasopressin and oxytocin agonists.

Synthesis and structure-activity investigation of novel vasopressin hypotensive peptide agonists

We report the solid phase synthesis and vasodepressor potencies of the novel hypotensive peptide [1(-beta-mercapto-beta,beta-pentamethylene propionic acid)-2-O-ethyl-D-tyrosine, 3-arginine, 4-valine] arginine vasopressin, d(CH2)5[D-Tyr(Et)2, Arg3, Val4]AVP (A), its related Lys3 (B), Tyr-NH(9)2 (C), [Lys3, Tyr-NH(9)2 (D) analogs and in a preliminary structure-activity study of positions 2-4 and 7-9, 24 analogs (1-24) of A-C. Peptides 1-6, 9-14 have the following single substituents at positions 2, 3, 4, 8 and 9 in (A): 1, D-Tyr(Me)2; 2, L-Tyr(Et)2; 3, Orn3; 4, N-Me-Arg3; 5, Glu3; 6, Arg4; 9, D-Arg8; 10, Eda9; 11, Arg-NH(9)2; 12, Ala-NH(9)2; 13, desGly9; 14, desGly-NH(9)2. Peptides 15 and 16 are analogs of B which possess the following single modifications: 15, Arg-NH(9)2; 16, desGly9. Peptides 7 and 8 are analogs of (C) with the following single modification: 7, Gln4; 8, Lys8. Peptides 17-24 are analogs of A possessing the following multiple modifications: 17, [Sar7, Eda9]; 18, [Arg7, Eda9]; 19, [Arg7, Eda9<--Tyr10]; 20, [Arg4, Arg-NH(9)2]; 21, [Ile4, desGly9]; 22, [Arg4, desGly9]l; 23, [Arg7, desGly9]; 24, [Arg7, Lys8, desGly9]. All 24 new peptides were evaluated for agonistic and antagonistic activities in in vivo antidiuretic (V2-receptor), vasopressor (V1a-receptor) and in in vitro (no Mg2+) oxytocic (OT-receptor) assays and like the parent peptides (A-D) (Chan et al. Br. J. Pharmacol. 1998; 125: 803-811) were found to exhibit no or negligible activities in these assays. Vasodepressor potencies were determined in anesthetized male rats with baseline mean arterial blood pressure maintained at 110-120 mmHg. The effective dose (ED), in microg 100 g(-1) i.v., required to produce a vasodepressor response of 5 cm2, area under the vasodepressor response curve (AUC) during the 5-min period following the injection of the test peptide, was determined. Therefore, the EDs measure the relative vasodepressor potencies of the hypotensive peptides. The following ED values were obtained for A-D and for peptides 1-24: A, 4.66; B, 5.75; C, 10.56; D, 11.60; 1, approximately 20; 2, approximately 30; 3, 6.78; 4, non-detectable (ND); 5, ND; 6, approximately 32; 7, ND; 8, 8.67; 9, ND; 10, 2.43; 11, 3.54; 12, 10.57; 13, 4.81; 14, ND; 15, 4.47; 16, 9.78; 17, 5.72; 18, 1.10; 19, 1.05; 20, 10.41; 21, 9.13; 22, approximately 33; 23, 3.01; 24, 1.71. A is clearly the most potent of the four original hypotensive peptides A-D. These data provide insights to which modification of A enhance, retain or abolish hypotensive potencies. Six of the new hypotensive peptides are significantly more potent than A. These are peptides 10, 11, 18, 19, 23 and 24. Peptide 19, a radioiodinatable ligand, is ten times more potent than C or D. The Gln4 modification of C and the N-Me-Arg3, Glu3, D-Arg8 and desGly-NH(9)2 modifications of A abolished hypotensive potency. By contrast, the Eda9, Arg-NH(9)2, [Sar7, Eda9], [Arg7, Eda9<- -Tyr10], [Arg7, desGly9], [Arg7, Lys8, desGly9] modifications of A all led to enhancements of hypotensive potency. This initial structure-activity exploration provides useful clues to the design of (a) more potent vasodepressor peptides and (b) high affinity radioiodinatable ligands for the putative AVP vasodilating receptor. Some of the peptides here may be of value as pharmacological tools for studies on the complex cardiovascular actions of AVP and may lead to the development of a new class of anti-hypertensive agents.

Discovery and design of novel vasopressin hypotensive peptide agonists

This presentation will trace the serendipitous discovery of novel vasopressin (VP) hypotensive agonists d(CH2)5[D-Tyr(Et)2,X3]VAVP (where X = Arg, Lys). These peptides were uncovered as part of an ongoing program aimed at the design of potent and selective VP antidiuretic (V2 receptor) antagonists. We will also present highlights of our subsequent preliminary studies seeking (i) to design high affinity radioiodinatable ligands for the localization and characterization of the putative VP vasodilatory (V1c?) receptor; (ii) to identify the structural features of selective and non-selective cyclic and linear VP and oxytocin (OT) antagonists of the V2 receptor, the vascular (V1a) receptor and of the uterine (OT) receptor required for hypotensive agonism and; (iii) to enhance hypotensive potency. These novel VP hypotensive agonists could serve as valuable research tools in studies on the roles of VP in blood pressure regulation and may also lead to the development of a new class of therapeutically useful antihypertensives.

The D136A mutation of the V2 vasopressin receptor induces a constitutive activity which permits discrimination between antagonists with partial agonist and inverse agonist activities

The substitution, in the human V2 vasopressin receptor, of the aspartate at position 136 by alanine leads to agonist-independent activation of this mutant V2 receptor. Pharmacological studies of the D136A V2 receptor helped us in characterizing different V2 receptor antagonists. SR-121463A and OPC-31260, two non-peptide antagonists, behaved as inverse agonists, while two cyclic peptides d(CH2)5[D-Tyr(Et)2,-Val4,Tyr-NH(2)9]AVP and d(CH2)5[D-Ile2,Ile4,Tyr-NH(2)9]AVP known to be V2 antagonists, demonstrated clear partial agonist properties. The finding of a constitutively activated human V2 receptor represents a useful tool in characterizing V2 receptor antagonist ligands.

Characterization of oxytocin actions in guinea-pig isolated uterine artery: the effect of pregnancy

While the contractile effect of oxytocin on uterine artery has been reported, little is known about whether pregnancy affects the responsiveness of this artery to oxytocin. If it does, is it a consequence of changed endothelial function, as has been proposed for some other vasoconstrictors. Furthermore, the receptor subtypes involved in oxytocin action on uterine artery has not been yet determined. Therefore the purposes of this study were to (1) determine the receptor subtypes involved in oxytocin action in non-pregnant and pregnant guinea-pig uterine artery and to (2) determine whether possible changes in uterine artery sensitivity to oxytocin during pregnancy are due to altered endothelial function. Therefore, the effect of oxytocin on non-pregnant and pregnant guinea-pig uterine arterial rings with and without endothelium was investigated. In non-pregnant guinea-pig uterine artery oxytocin induced contraction (pEC50 = 7.63) with greater potency than in pregnant guinea-pig uterine artery (pEC50 = 7.17). Removal of the endothelium did not affect oxytocin-induced contractions, regardless of the pregnancy status. The uterine arteries did not respond to [Thr4, Gly7]oxytocin. In the preparations studied, [d(CH2)5Tyr(Me)2]vasopressin and [d(CH2)5, D-Ile2, Ile4]vasopressin antagonized oxytocin action with the following pKB values ([d(CH2)5Tyr(Me)2]vasopressin versus [d(CH2)5, D-Ile2, Ile4]vasopressin): 8.24 versus 7.29 and 8.11 versus 7.17 for non-pregnant guinea-pig uterine artery with and without endothelium, respectively; 8.39 versus 7.25 and 8.35 versus 7.25 for pregnant guinea-pig uterine artery with and without endothelium, respectively. We suggest that, in uterine arteries, oxytocin induces contraction by activation of vasopressin V1A receptors. The potency of oxytocin in uterine artery is decreased during pregnancy and this is not associated with altered endothelial function.

Effect of oxytocin as a partial agonist at vasoconstrictor vasopressin receptors on the human isolated uterine artery

1. The effect of oxytocin on endothelium-intact and endothelium-denuded segments of the human uterine artery rings was investigated. 2. In both types of preparation oxytocin induced contraction of human uterine artery with similar potency and efficacy (pEC50 values: 6.95 +/- 0.05 vs 7.06 +/- 0.01; maximal response values: 61 +/- 4.1% vs 63 +/- 5.1% for arteries with and without endothelium, respectively). 3. In contrast, human uterine arteries, both intact and denuded of endothelium, did not respond to the addition of the selective oxytocin receptor agonist, [Thr4, Gly7]oxytocin (10 nM(-1) microM). 4. The vasopressin receptor antagonists, [d(CH2)5Tyr(Me)]AVP (10-100nM) and [d(CH2)5,D-Ile2,Ile4]AVP (300 nM-3 microM) produced parallel rightward shifts of the curves for oxytocin. The Schild plots constrained to a slope of unity gave the following -log K(B) values: [d(CH2)5Tyr(Me)] AVP vs [d(CH2)5,D-Ile2,Ile4] AVP 9.24 vs 6.91 and 9.26 vs 6.84 for human uterine artery with intact and those denuded of endothelium, respectively. In contrast, in both types of preparations the oxytocin receptor antagonist, [d(CH2)5Tyr(OMe), 2Orn8]vasotocin (1 microM), did not significantly affect oxytocin-induced contractions. 5. The calculated pK(A) values for oxytocin itself also did not differ between preparations: 6.56 and 6.43 for human uterine artery with and without endothelium, respectively. In both types of preparations, the receptor reserve (K(A)/EC50) was close to unity (intact vs denuded: 3.9 vs 3.0). 6. It is concluded that, in human uterine artery, oxytocin induces contractions that are not modulated by the endothelium. It is likely that oxytocin acts as a partial agonist on human uterine artery, regardless of the endothelial condition. On the basis of differential antagonists affinity and affinity of oxytocin itself, it is probable that receptors involved in oxytocin-induced contraction in human uterine arteries belong to the V(1A) vasopressin receptors.

Molecular pharmacology of V1a vasopressin receptors

1. Vasopressin, a mammalian neurohypophysial peptide hormone, has diverse physiological actions. 2. Pharmacological studies, using a range of mammalian tissues, have identified three subtypes of vasopressin receptor. 3. The V1a subtype of vasopressin receptor is widely distributed and mediates many central and peripheral actions of vasopressin. 4. The development of subtype-selective vasopressin analogues has provided valuable tools for pharmacological and physical studies of the V1a receptor protein. 5. Pharmacological differences indicate species heterogeneity in the characteristics of V1a receptors and in the expression of hepatic V1a receptors. 6. The cloning of neurohypophysial hormone receptor proteins allows structural and functional comparison of the V1a vasopressin receptors with other G-protein-coupled receptors.

Characterization of arginine vasopressin actions in human uterine artery: lack of role of the vascular endothelium

1. The effect of arginine vasopressin (AVP) on human uterine artery rings, both intact and denuded of endothelium, was investigated. 2. Initially, AVP (63 pM-32 nM) induced concentration-dependent contraction of human uterine artery (pD2 = 8.92 +/- 0.01). Removal of the endothelium did not affect the concentration-response curve for AVP (pD2 = 8.83 +/- 0.03). 3. In contrast, human uterine arteries, both intact and denuded of endothelium, did not respond to the addition of 1-desamino-8-D-arginine vasopressin (dDAVP, 1 nM-1 microM). 4. In both types of preparations, [d(CH2)5Tyr(Me)AVP (1-10 nM) and [d(CH2)5,D-Ile2,Ile4]AVP (300 nM-3 microM) produced parallel rightward shifts of the curves for AVP. The Schild plots constrained to a slope of unity gave the following -log KB values: [d(CH2)5Tyr(Me)]AVP vs. [d(CH2)5,D-Ile2,Ile4]AVP 9.66 vs. 6.69 and 9.61 vs. 6.80 for human uterine artery, intact and denuded of endothelium, respectively. 5. The pKA values for AVP itself also did not differ between preparations: 6.56 and 6.43 for human uterine artery with and without endothelium, respectively. In both types of preparations, the receptor reserve (KA/EC50) was considerably greater than unity (intact vs. denuded: 228 vs. 244). 6. It is concluded that, in human uterine artery, AVP induces contractions that are not modulated by the endothelium. It is likely that AVP acts as a full agonist on human uterine artery, regardless of the endothelial condition. On the basis of differential antagonists affinity and affinity of AVP itself, it is probable that vasopressin receptors involved in AVP-induced contraction in human uterine arteries belong to the V1a or V1a-like subtype.

Effect of pregnancy on vasopressin-mediated responses in guinea-pig uterine arteries with intact and denuded endothelium

The effect of pregnancy on vasopressin-induced contraction of guinea-pig uterine arterial rings was investigated. Initially, vasopressin induced contraction (pD2 = 9.14) in pregnant guinea-pig uterine artery with greater potency than in non-pregnant guinea-pig uterine artery (pD2 = 8.77). Removal of the endothelium did not affect vasopressin-induced contractions, regardless of pregnancy status. In all types of preparations, [d(CH2)5Tyr(Me)2]vasopressin (10-100 nM) and [d(CH2)5,D-Ile2,Ile4]vasopressin (300 nM-3 microM) produced parallel rightward shifts of the curves for vasopressin. The Schild plots constrained to a slope of unity gave the following -log KB values: [d(CH2)5Tyr(Me)2]vasopressin vs. [d(CH2)5,D-Ile2,Ile4]vasopressin 8.74 vs. 6.82 and 8.50 vs. 6.72 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively; 8.38 vs. 6.49 and 8.36 vs. 6.75 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. The pKA values for vasopressin itself also did not differs between preparations: 6.49 and 6.55 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively; 6.48 and 6.52 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. The receptor reserve (KA/EC50) was significantly greater in preparations taken from pregnant than from non-pregnant animals. It is concluded that vasopressin-induced contractions of guinea-pig uterine artery are not modulated by the endothelium, regardless of pregnancy status. The receptor reserve for vasopressin in guinea-pig uterine artery is increased during pregnancy, that is not related to the changes of vasopressin receptor affinity for vasopressin. It is probable that vasopressin receptors involved in vasopressin-induced contraction of all types of vessels studied belong to the V1A-like subtype.

Indomethacin and arginine vasopressin interaction in the fetal kidney: a mechanism of oliguria

OBJECTIVE

Indomethacin has been shown to cause fetal oliguria in humans and animals. This study was designed to test the hypothesis that indomethacin-induced fetal oliguria is mediated through the renal action of arginine vasopressin.

STUDY DESIGN

Twenty-seven late-gestation (129 +/- 4 days [mean +/- SE]) chronically catheterized fetal sheep were studied. After a 1-hour control period fetal responses to indomethacin, 0.05 mg/kg given intravenously, followed by a 4-hour maintenance infusion (n = 9), were compared with an identical indomethacin infusion plus an arginine vasopressin V2-receptor antagonist (d[CH2]1(5), D-Phe2, Ile4,Arg8,Ala9)-VP (n = 8) or vehicle alone (n = 10). Fetal arterial and venous pressures, heart rate, and urinary flow were measured continuously.

RESULTS

Fetal urinary flow rate (p < 0.0001) and free water clearance (p = 0.004) fell in response to indomethacin alone, but the addition of the arginine vasopressin V2-receptor antagonist blocked indomethacin's oliguric and free water effect. Urinary osmolality and sodium increased in both indomethacin and indomethacin+arginine vasopressin V2-receptor antagonist groups compared with vehicle (p < 0.05). Fetal arterial pressure increased in response to indomethacin, and the addition of the arginine vasopressin V2-receptor antagonist potentiated this response (p = 0.007).

CONCLUSIONS

These results suggest that (1) fetal oliguria secondary to indomethacin is mediated through the stimulation of the renal arginine vasopressin V2-receptor and (2) prostaglandin synthesis inhibition may play a role in renal tubular sodium handling. In addition, the arginine vasopressin V2-receptor plays a role in ameliorating the hypertensive response to indomethacin. We speculate that indomethacin stimulates circulating arginine vasopressin levels and enhances peripheral arginine vasopressin effects in the fetus, resulting in oliguria and hypertension.

Differential effects of V2 vasopressin agonist and antagonists on blood pressure regulation in normotensive (WKY) and spontaneously hypertensive (SHR) rats

The effects of arginine vasopressin analogs with V2 agonistic and antagonistic properties on blood pressure (BP) and heart rate (HR) were compared in conscious, spontaneously hypertensive (SHR) and normotensive (WKY) rats under resting conditions and after administration of phenylephrine (Phe) and sodium nitroprusside (SN). In WKY rats, resting BP and HR were not significantly affected during intravenous (i.v.) infusion of dVDAVP, (V2 agonist; 200 pg/kg/min), d(CH2)5 (D-Ile2,Abu4]AVP (V2 antagonist 1; weak V1 antagonist; V2/V1 ratio = 29; 0.6 microgram/kg/min), d(CH2)5[D-Ile2,Ile4,AlaNH2]AVP (V2 antagonist 2; very weak V1 antagonist; V2/V1 ratio = 83; 0.6 microgram/kg/min) and combined infusion of V2 agonist and V2 antagonist 2. Under resting conditions BP and HR were not affected in WKY by any of the treatments. In SHR rats BP and HR were significantly decreased by V2 antagonist 2 infused alone or in combination with V2 agonist. In WKY but not in SHR V2 agonist without and with prior V2 receptors blockade significantly augmented bradycardia associated with a maximum increase of the systolic blood pressure after Phe administration. Significant differences were found between SHR and WKY in SN-induced changes of HR and BP after administration of V2 agonist and antagonists. The results suggest that circulating vasopressin may modify the baroreflex by interaction with receptors which are stimulated by V2 agonist but are different from the classical V2 receptors. The study supports evidence for differential effects of vasopressin analogs on blood pressure and blood pressure-heart rate relations in WKY and SHR.

Purification of the hepatic vasopressin receptor using a novel affinity column

A vasopressin receptor was purified, using a novel affinity column, from rat liver plasma membranes treated with guanosine 5'-(3-O-thio)triphosphate and solubilized with 0.8% cholate. Incubation of the membranes with the GTP analogue resulted in a dissociation of the receptor-guanine nucleotide regulatory protein complex. This manipulation, although resulting in a low-affinity state of the receptor, facilitated purification. The solubilized receptor was assayed using a new reconstitution procedure in which the soluble extracts were inserted into lipid vesicles composed of phosphatidylcholine and phosphatidylinositol. The receptor was purified by sequential chromatography on Q-Sepharose and hydroxyapatite. The use of a novel affinity column, a V1-vasopressin antagonist-agarose, resulted in a near-homogeneous preparation of a protein which exhibited an Mr = 58,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Autoradiography of purified receptor, as well as crude membrane preparations cross-linked to [125I]arginine vasopressin, also revealed a protein band with an approximate Mr = 58,000. These findings indicate that V1-antagonist affinity chromatography should be useful for purifying adequate amounts of the receptor for studies of structure and function.

Effects of vasopressin and other neuropeptides on rostral medullary sympathoexcitatory neurons 'in vitro'

Neurons with intrinsic pacemaker activity and presumed sympathoexcitatory function were recorded in rat tissue slices within the confines of the rostroventrolateral reticular nucleus (RVL). These cells were excited in dose-dependent fashion by arginine vasopressin (AVP, 10(8)-10(6) M) but not by oxytocin (up to 10(7) M). The effect of AVP was mimicked by the V1-selective agonist [Phe2,Orn8]vasotocin (VT) (1 microM) but not by the V2-agonist [Val4,D-Arg8]vasopressin (VP) (1.9 microM). The effect of AVP (10(-7) M) was completely blocked by SKF 101926 (10(7) M), a non-selective antagonist and by d(CH2)5[Tyr(Me)2]AVP, a V1-selective antagonist but was unaffected by the V2-selective antagonist d(CH2)5[D-Ile2,Ile4,Ala-NH2 9]AVP. These cells were also activated by thyrotropin-releasing hormone (TRH) (10(-7)-10(-6) M), calcitonin gene-related peptide (CGRP) (4 X 10(-8) M), substance P, (10(-6) M), neuropeptide Y (NPY) (10(-8) M) and inhibited by Met-enkephalin (10(-6) M) and morphine (2 mM). Corticotropin-releasing factor (CRF) (10(-7) M) and angiotensin II (10(-6) M) were ineffective. In conclusion, RVL pacemaker neurons have vasopressin receptors reminiscent of the V1 (vascular and pressor) subtype. Their pacemaking activity is modulated by low doses of several other peptides also known to produce large vasomotor effects after introduction into the cerebroventricular space.

Novel linear antagonists of the antidiuretic (V2) and vasopressor (V1) responses to vasopressin

We report the solid phase synthesis of a series of 16 linear analogues of the cyclic antagonist of the antidiuretic (V2) and the vasopressor (V1) responses to arginine vasopressin (AVP), d(CH2)5[D-Tyr(Et)2, Val4]AVP(A). Peptide 1, the linear precursor of (A), (CH2)5(SH)-CH2-CO-D-Tyr(Et)-Phe-Val-Asn-Cys-Pro-Arg-Gly-NH2 was modified at position six with alpha-L-aminobutyric acid (Abu) to give peptide 2. Further modifications of the Abu6 analogue (No. 2) at position one by substituting cyclohexylacetic acid (Caa), cyclohexylpropionic acid (Cpa), 1-adamantaneacetic acid (Aaa), phenylacetic acid (Phaa), tert.-butylacetic acid (t-Baa), isovaleric acid (Iva), propionic acid (Pa), L-penicillamine (P), tert.-butoxycarbonyl (Boc) or omitting any substituent at this position, and/or in combination with Arg-NH2(9), Ala-NH2(9), D-Arg8-Arg-NH2(9), and desGly9 modifications yielded the remaining 14 peptides. All 16 peptides were examined for agonistic and antagonistic potencies in AVP V2 and V1 assays in rats. Apart from the Cpa analogue and the analogue lacking any substituent in the 1-position, all exhibit substantial V2 and V1 antagonism. A number are as potent as (A) as V2 antagonists. With an anti-V2 pA2 = 8.11 +/- 0.07, Aaa-D-Tyr(Et)-Phe-Val-Asn-Abu-Pro-Arg-Arg-NH2 (No. 6) is as potent as any cyclic AVP V2 antagonist reported to date. The PaI analogue of No. 6 exhibits promising anti-V2/anti-V1 selectivity. These findings prove conclusively that a ring structure is not a requirement for recognition of or for binding to AVP V2 or V1 receptors. This discovery thus offers a promising new approach to the design of peptide and non-peptide antagonists of AVP and perhaps also to other cyclic peptides such as somatostatin, atrial-natriuretic factor, insulin, and the recently discovered endothelin. Some of these linear antagonists may be of value as pharmacological tools and as therapeutic agents.